The lady’s slipper orchid (Cypripedium fargesii) does not look well. Its red and yellow flowers are nestled among two large leaves, both covered in unsightly black splotches. These look like the signs of a fungal infection, but they’re not. This orchid is deceptive, not diseased. It produces the black spots itself and in doing so, it lures in flat-footed flies that feed on fungus. The flies, duped by the orchid’s false spots, pick up pollen and spread it to another flower. By appearing infected, the orchid reproduces.

There are many species of lady’s slipper orchids, but C.fargesii is an exceptionally rare variety. This critically endangered flower is found only in southwestern Chinese mountains. Zong-Xin Ren from the Chinese Academy of Sciences spent four consecutive summers on Yaoshan Mountain, almost 3,000 metres above sea level, studying a hundred or so of these rare flowers.

It’s fairly easy to work out what pollinates a lady’s-slipper orchid. When insects land and enter the flower, they have to walk through a circular route before escaping through some fixed exits. Watch the exits, and you find the pollinators. This defined path takes the insects past the flower’s female organ – the stigma – and its male ones – the anthers. These deposit clumps of pollen onto the insects’ heads and bodies, which they carry to the stigma of the next flower they visit.

The lady’s slippers are generally pollinated by bees but C.fargesii is different. Over many hours of observation, the only insects that Ren ever saw leaving the flowers were flat-footed flies. Ren captured four of them and when he peered at them under an electron microscope, he saw pollen grains from the orchid, and spores from a fungus called Cladosporium. This fungus infects leaves and fruits, and when it does so, it produces black mould spots. The purpose of the orchid’s black splotches was becoming clear.

Ren also analysed the orchid’s scent, an unpleasant fragrance reminiscent of rotting leaves. He found that the flower produces over 50 aromatic molecules that are found in other flowers, but three unusual ones that are common to Cladosporium moulds.

Ren thinks that these fungal odours lure the flies to the flower in the first place. Once there, the black spots add to the illusion. This ruse continues at a microscopic level. At the centre of each spot are hairs (‘trichomes’) that have evolved bulges and waists that mimic chains of fungal spores. The flower mimics the fungus in both appearance and smell.

Darwin himself was fascinated by the evolution of orchids but he failed to realise the family’s massive capacity for deception. While many species reward their pollinators with edible pollen or sweet nectar, the majority – including the lady’s slippers – are liars that provide no such remuneration. Some species lure bees in with the promise of sex, or wasps with the smell of fresh meat. In fact, deception may explain why the orchids are among the most diverse of flowering plants, with anywhere between 22,000 and 26,000 species on record. They have evolved to con an ever greater variety of pollinators.

But deception can go both ways. C.fargesii recruits fungus-eating insects to spread its pollen, by looking and smelling like a fungus, but offering no nectar. It has a mirror-image counterpart – a group of fungi called Monilinia that infect and kill plants. These fungi recruit flower-pollinating insects to spread their spores, by looking and smelling like flowers, and offering nectar.

There are 9 Comments. Add Yours.

What’s astounding is that it isn’t merely the black blotches or scent that has been emulated, the flower has mimicked microscopic parts as well! I am increasingly becoming more fond of orchids.

Sam W
April 19, 2011

Are the aromatic molecules the same? How common is that? How would the orchid get the ability to produce these molecules on a molecular level? Is this relatively likely to happen by chance? (I have no idea how complex or different from each other aromatic compounds are…)

Sam, several of the orchids have nature-identical aromatic lure chemicals to mimic sex pheromones of female insects. It seems incredible that they can do this considering the millions of possible variants on organic chemicals. I am not surprised that this one has three that are identical to Cladosporium aromatics, boggled but not surprised. I would recommend looking at Ophrys research. Other orchids are equally strange. Rhizanthella gardneri never sees the light of day and flowers underground. It can be found when the ground cracks and the smell of the flower detected.

The Stapeliads have a range of spectacular and very smelly flowers. They are famous for “carrion flowers” that mimic rotten meat in appearance and smell. It has recently been found that they produce nature-identical aromatic chemicals that are specific enough to determine what type of ickiness they are imitating. They attract specific flies. Some species imitate dung, either carnivore, omnivore or vegetarian, others rotting meat or fermenting urine. There is one species from near the sea that imitates rotting fish.

The old Asclepiadaceae that contains the Stapeliads (now part of the Apocynaceae) are often regarded as the dicotyledonous counterpart to the orchids as they have similarly intricate, variable flowers and also have their pollen packaged in lumps called pollinia.

Thanks for the amazing orchid info, more mind-boggling.

Chris
April 19, 2011

Fascinating – but couldn’t this backfire for the plant? It’s attracting flies which could infect it with the fungus (and other types too, depending on how much variety these flies like in their diet).
I guess the orchid must be resistant although I don’t see any mention of that in the paper. I’d be interested to know about the resistance – are closely related plants resistant or not?
Guess I’ll do some searching. After I’ve done my real work for the day….

Brian Too
April 19, 2011

Freaky, funky fungus fun!

lr
April 21, 2011

It may or not be relevant that similar leaf spots occur in quite a few, mostly unrelated orchids; some like C. fargesii grow in deep shade. Others, eg, Dactylorhiza fuchsii, in open wetlands. No idea if there’s a consistent pattern of pollinators across such species.

Steve
April 24, 2011

This reminds me of Maxillaria pseudoreichenheimii that have silvery white spots on their leaves. This also looks like a disease, though I’m not enough of a plant pathologist to guess what kind. No idea of the pollinator.

Fred
May 2, 2011

Aren’t the fungal spores 1/20th the size of the leaf hairs? I doubt the flies would be fooled by such a discrepancy.

Carol
May 22, 2011

Steve, A grower of Max. pseudo-reichenheimii told me the spots on the leaves mimic bird droppings.

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